I'm sure valve get's a portion of the funds from devs who use cloud, vac, etc,. in addition to their own games. If I'm reading your question correctly though they do not have any licensing or certification fees associated with putting a game on the steam platform unlike xbla or psn which is why a lot of indy devs go the steamroute first instead of throwing a game on consoles....

So, if they get a 'cut', where does the rest of the money go? Co-developers or something?

Someone once said, if you make a game and put it on Steam to be sold, you get 30%, Valve gets 70%. If the same thing would be on retail, then you get like 10%, game distributors (like EA, ACtivision) gets the 90%. While these numbers are really not exactly true, you get the idea.

Valve provides the service - Store, online distribution, Steamworks (achievements, VAC, Steam cloud) that developers can use. They even licence Source Engine if someone is willing to buy a licence.

Making things click takes money, bandwidth, etc. Valve got the ready package, Steam that handles that. After all, since they are a company, they try to make profit alongside of things. Just like any other company.

Their own games are just portion of the thing that Steam sells and these days they just do those for promoting Steam and for fun.